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Improved Selective Acknowledgment Scheme for TCP

Improved Selective Acknowledgment Scheme for TCP. Raj Kettimuthu & Bill Allcock Globus Alliance / Argonne National Laboratory. Introduction. Improved Selective Acknowledgment (ISACK) Scheme addresses limitations of TCP selective acknowledgment (SACK) mechanism

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Improved Selective Acknowledgment Scheme for TCP

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  1. Improved Selective Acknowledgment Scheme for TCP Raj Kettimuthu & Bill Allcock Globus Alliance / Argonne National Laboratory

  2. Introduction • Improved Selective Acknowledgment (ISACK) Scheme addresses limitations of TCP selective acknowledgment (SACK) mechanism • SACK can convey information about only 4 noncontiguous blocks of data received • Sender might have to unnecessarily retransmit some packets • ASACK uses both SACK and ISACK to give optimal performance IC 2004

  3. Background • TCP provides connection-oriented, reliable byte stream service. • Provides reliability by assigning a sequence number to each octet transmitted and by requiring a positive ACK • The acknowledgment mechanism is cumulative • TCP provides flow control • Return a “window” with every ACK indicating a range of acceptable sequence numbers IC 2004

  4. Background • Routers and slower links between sender and receiver may cause congestion • Slow start, congestion avoidance, fast retransmit and fast recovery to deal with congestion • Congestion window (cwnd) and slow start threshold size (ssthresh) for each connection • Sender transmits up to min(congestion window, advertised window) IC 2004

  5. Congestion control algorithm • Cwnd initialized to one segment and ssthresh to a large value • If (cwnd <= ssthresh), TCP performs slow start; else it performs congestion avoidance • Slow start – cwnd begins at one segment and incremented by one segment for every ACK • Congestion avoidance – increase in cwnd is at most one segment per RTT IC 2004

  6. Congestion control algorithm • Congestion indicated by timeout or reception of 3 consecutive duplicate ACKs • When a timeout occurs, set ssthresh to max(window/2, 2) and cwnd to one segment • Note: window = min(cwnd, advertised window) • On receiving 3 consecutive dupacks, fast retransmit and fast recovery are performed IC 2004

  7. Congestion control algorithm • Fast retransmit – retransmits apparently missing segment, set ssthresh = max(window/2, 2), cwnd = ssthresh + 3 and enter fast recovery • Inflates cwnd by the number of segments that have left the network and that the other end has cached • Receipt of dupacks tells TCP more than just a packet has been lost – data is still flowing between the two ends IC 2004

  8. Congestion control algorithm • Fast recovery – increments cwnd by segment size each time a dupack arrives and transmits a packet (if allowed) • When next ACK arrives that acknowledges the retransmitted data, set cwnd = ssthresh and enter congestion avoidance • TCP Reno includes this congestion control algorithm IC 2004

  9. Behavior of TCP Reno IC 2004

  10. TCP New-Reno • When a partial ACK is received, it retransmits the first unacknowledged segment in the window and remains in fast recovery • Remains in fast recovery until all of the data outstanding when fast recovery was initiated has been acknowledged • When multiple segments are lost from a single window, it recovers without a timeout, retransmitting one lost segment per RTT IC 2004

  11. SACK • Retransmitting one lost segment per RTT is still slow • SACK helps recover faster by providing additional information about the state of congestion • Uses two new TCP options IC 2004

  12. SACK IC 2004

  13. Limitations of SACK • SACK option that specifies “n” noncontiguous blocks will have a length of “8*n+2” bytes • TCP options space – 40 bytes • SACK can specify a maximum of 4 blocks • SACK is often used with timestamp option, reducing the number of blocks to 3 • Introduction of new options may reduce it further IC 2004

  14. Limitations of SACK IC 2004

  15. ISACK • For each noncontiguous block, ISACK sends the offset of the left edge from the 32-bit “(cumulative) Acknowledgment Number” field • Uses 2 new TCP options • Enabling option sent in SYN segment IC 2004

  16. ISACK option IC 2004

  17. ISACK option • “Offset” field specifies the number of bits used to represent the offsets of each left edge • Value is given by ceil(log2(maxoffset)); maxoffset is the largest among the offsets • “Size” field specifies the number of bits used to represent the size of each block • Value is given by ceil(log2(maxsize)); maxsize is the size of the largest block IC 2004

  18. Behavior of ISACK IC 2004

  19. Behavior of ISACK • Maxoffset = 4500 • Number of bits used to represent each offset = ceil(log2(4500)) = 13 • Maxsize = 500 • Number of bits used to represent the size of each block = ceil(log2(500)) = 9 • Total number of bits required by ISACK option to specify the 5 noncontiguous blocks = 8(Kind) + 8(Length) + 8(Offset) + 8(Size) + 5*13(offsets) + 5*9(sizes) = 142 bits(18 bytes) IC 2004

  20. ASACK • ISACK imposes a little more processing overhead than does SACK • Use ISACK only when SACK can not convey the information • ASACK dynamically switches between SACK and ISACK to give optimal performance IC 2004

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